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<p>Declares a named variable as a reference, that is, an alias to an already-existing object or function.
</p>
<table id="toc" class="toc"><tr><td>
<div id="toctitle"><h2>Contents</h2></div>
<ul>
<li class="toclevel-1 tocsection-1">
<a href="reference.html%23Syntax"><span class="tocnumber">1</span> <span class="toctext">Syntax</span></a>
<ul>
<li class="toclevel-2 tocsection-2"><a href="reference.html%23Lvalue_references"><span class="tocnumber">1.1</span> <span class="toctext">Lvalue references</span></a></li>
<li class="toclevel-2"><a href="reference.html%23Rvalue_references"><span class="tocnumber">1.2</span> <span class="toctext">Rvalue references</span></a></li>
<li class="toclevel-2 tocsection-4"><a href="reference.html%23Dangling_references"><span class="tocnumber">1.3</span> <span class="toctext">Dangling references</span></a></li>
</ul>
</li>
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<h3>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Syntax">edit</a>]</span> <span class="mw-headline" id="Syntax">Syntax</span>
</h3>
<p>A reference variable declaration is any simple declaration whose <a href="declarations.html" title="cpp/language/declarations">declarator</a> has the form
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<td> <code><b>&amp;</b></code> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">declarator</span>
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<td> (1)
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<td> <code><b>&amp;&amp;</b></code> <span class="t-spar">attr</span><span class="t-mark">(optional)</span> <span class="t-spar">declarator</span>
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<td> (2)
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<td> <span class="t-mark-rev t-since-cxx11">(since C++11)</span>
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<span class="t-li">1)</span> <b>Lvalue reference declarator</b>: the declaration <span class="t-c"><span class="mw-geshi cpp source-cpp">S <span class="sy3">&amp;</span> D<span class="sy4">;</span></span></span> declares <code>D</code> as an <i>lvalue reference</i> to the type determined by <span class="t-spar">decl-specifier-seq</span> <code>S</code>.</div>
<div class="t-li1">
<span class="t-li">2)</span> <b>Rvalue reference declarator</b>: the declaration <span class="t-c"><span class="mw-geshi cpp source-cpp">S <span class="sy3">&amp;&amp;</span> D<span class="sy4">;</span></span></span> declares <code>D</code> as an <i>rvalue reference</i> to type determined by <span class="t-spar">decl-specifier-seq</span> <code>S</code>.</div>
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<td>  <span class="t-spar">declarator</span>
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<td> -
</td>
<td>  any <a href="declarations.html" title="cpp/language/declarations">declarator</a> except another reference declarator (there are no references to references)
</td>
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<td>  <span class="t-spar">attr</span><span class="t-mark-rev t-since-cxx11">(C++11)</span>
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<td> -
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<td>  optional list of <a href="attributes.html" title="cpp/language/attributes">attributes</a>
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<p>A declaration of a reference (except for class members and function parameters) is required to contain an initializer: see <a href="reference_initialization.html" title="cpp/language/reference initialization">reference initialization</a> which must identify a valid object or function.
</p>
<p>There are no references to <span class="t-c"><span class="mw-geshi cpp source-cpp"><span class="kw4">void</span></span></span> and no references to references.
</p>
<p>Reference types cannot be <a href="cv.html" title="cpp/language/cv">cv-qualified</a> at the top level; there is no syntax for that in declaration, and if a qualification is introduced through a typedef, decltype, or template type argument, it is ignored.
</p>
<p>References are not objects; they do not necessarily occupy storage, although the compiler may allocate storage if it is necessary to implement the desired semantics (e.g. a non-static data member of reference type usually increases the size of the class by the amount necessary to store a memory address)
</p>
<p>Because references are not objects, there are no arrays of references, no pointers to references, and no references to references:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw4">int</span><span class="sy3">&amp;</span> a<span class="br0">[</span><span class="nu0">3</span><span class="br0">]</span><span class="sy4">;</span> <span class="co1">// error</span>
<span class="kw4">int</span><span class="sy3">&amp;</span><span class="sy2">*</span> p<span class="sy4">;</span> <span class="co1">// error</span>
<span class="kw4">int</span><span class="sy3">&amp;</span> <span class="sy3">&amp;</span>r<span class="sy4">;</span> <span class="co1">// error</span></pre></div></div>
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<p>It is permitted to form references to references through type manipulations in templates or typedefs, in which case the <i>reference collapsing</i> rules apply: rvalue reference to rvalue reference collapses to rvalue reference, all other combinations form lvalue reference:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="kw1">typedef</span> <span class="kw4">int</span><span class="sy3">&amp;</span>  lref<span class="sy4">;</span>
<span class="kw1">typedef</span> <span class="kw4">int</span><span class="sy3">&amp;&amp;</span> rref<span class="sy4">;</span>
<span class="kw4">int</span> n<span class="sy4">;</span>
lref<span class="sy3">&amp;</span>  r1 <span class="sy1">=</span> n<span class="sy4">;</span> <span class="co1">// type of r1 is int&amp;</span>
lref<span class="sy3">&amp;&amp;</span> r2 <span class="sy1">=</span> n<span class="sy4">;</span> <span class="co1">// type of r2 is int&amp;</span>
rref<span class="sy3">&amp;</span>  r3 <span class="sy1">=</span> n<span class="sy4">;</span> <span class="co1">// type of r3 is int&amp;</span>
rref<span class="sy3">&amp;&amp;</span> r4 <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span> <span class="co1">// type of r4 is int&amp;&amp;</span></pre></div></div>
<p>(this, along with special rules for <a href="template_argument_deduction.html" title="cpp/language/template argument deduction">template argument deduction</a> when <code>T&amp;&amp;</code> is used in a function template, forms the rules that make <span class="t-lc"><a href="../utility/forward.html" title="cpp/utility/forward">std::forward</a></span> possible)
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td>
</tr>
</table>
<h4>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Lvalue references">edit</a>]</span> <span class="mw-headline" id="Lvalue_references">Lvalue references</span>
</h4>
<p>Lvalue references can be used to alias an existing object (optionally with different cv-qualification)
</p>
<div class="t-example">
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a> s <span class="sy1">=</span> <span class="st0">"Ex"</span><span class="sy4">;</span>
    <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span> r1 <span class="sy1">=</span> s<span class="sy4">;</span>
    <span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span> r2 <span class="sy1">=</span> s<span class="sy4">;</span>
 
    r1 <span class="sy2">+</span><span class="sy1">=</span> <span class="st0">"ample"</span><span class="sy4">;</span> <span class="co1">// modifies s</span>
<span class="co1">//  r2 += "!"; // error: cannot modify through reference to const</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> r2 <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span> <span class="co1">// prints s, which now holds "Example"</span>
<span class="br0">}</span></pre></div></div>
<p><br> 
</p>
</div>
<p>They can also be used to implement pass-by-reference semantics in function calls:
</p>
<div class="t-example">
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="kw4">void</span> double_string<span class="br0">(</span><a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span> s<span class="br0">)</span>
<span class="br0">{</span>
    s <span class="sy2">+</span><span class="sy1">=</span> s<span class="sy4">;</span> <span class="co1">// 's' is the same object as main()'s 'str'</span>
<span class="br0">}</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a> str <span class="sy1">=</span> <span class="st0">"Test"</span><span class="sy4">;</span>
    double_string<span class="br0">(</span>str<span class="br0">)</span><span class="sy4">;</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> str <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p><br> 
</p>
</div>
<p>When a function's return type is lvalue reference, the function call expression becomes an <a href="value_category.html" title="cpp/language/value category">lvalue expression</a>:
</p>
<div class="t-example">
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="kw4">char</span><span class="sy3">&amp;</span> char_number<span class="br0">(</span><a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span> s, <a href="../types/size_t.html"><span class="kw100">std::<span class="me2">size_t</span></span></a> n<span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw1">return</span> s.<span class="me1">at</span><span class="br0">(</span>n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// string::at() returns a reference to char</span>
<span class="br0">}</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a> str <span class="sy1">=</span> <span class="st0">"Test"</span><span class="sy4">;</span>
    char_number<span class="br0">(</span>str, <span class="nu0">1</span><span class="br0">)</span> <span class="sy1">=</span> <span class="st0">'a'</span><span class="sy4">;</span> <span class="co1">// the function call is lvalue, can be assigned to</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> str <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p><br> 
</p>
</div>
 <table class="t-rev-begin">
<tr class="t-rev t-since-cxx11">
<td>
<h4> <span class="mw-headline" id="Rvalue_references">Rvalue references</span>
</h4>
<p>Rvalue references can be used to extend the lifetime of a modifiable temporary (note, lvalue references to const can <a href="reference_initialization.html" title="cpp/language/reference initialization">extend lifetimes</a> too, but they are not modifiable)
</p>
<div class="t-example">
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="co2">#include &lt;string&gt;</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a> s1 <span class="sy1">=</span> <span class="st0">"Test"</span><span class="sy4">;</span>
<span class="co1">//    std::string&amp;&amp; r1 = s1; // error: can't bind to lvalue</span>
 
    <span class="kw4">const</span> <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span> r2 <span class="sy1">=</span> s1 <span class="sy2">+</span> s1<span class="sy4">;</span> <span class="co1">// OK, lvalue ref to const extends lifetime</span>
<span class="co1">//    r2 += "Test"; // error: can't modify through reference to const</span>
 
    <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a><span class="sy3">&amp;&amp;</span> r3 <span class="sy1">=</span> s1 <span class="sy2">+</span> s1<span class="sy4">;</span> <span class="co1">// OK, rvalue ref extends lifetime</span>
    r3 <span class="sy2">+</span><span class="sy1">=</span> <span class="st0">"Test"</span><span class="sy4">;</span> <span class="co1">// this rvalue can be modified</span>
    <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> r3 <span class="sy1">&lt;&lt;</span> <span class="st0">'<span class="es1">\n</span>'</span><span class="sy4">;</span>
<span class="br0">}</span></pre></div></div>
<p><br> 
</p>
</div>
<p>More importantly, when a function has both rvalue reference and lvalue reference overloads, the rvalue reference overload binds to rvalues, while the lvalue reference overload binds to lvalues.
</p>
<div class="t-example">
<div class="t-example-live-link"><div class="coliru-btn coliru-btn-run-init">Run this code</div></div>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><span class="co2">#include &lt;iostream&gt;</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="sy3">&amp;</span> x<span class="br0">)</span>
<span class="br0">{</span>
     <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"lvalue reference overload f("</span> <span class="sy1">&lt;&lt;</span> x <span class="sy1">&lt;&lt;</span> <span class="st0">")<span class="es1">\n</span>"</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">const</span> <span class="kw4">int</span><span class="sy3">&amp;</span> x<span class="br0">)</span>
<span class="br0">{</span>
     <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"lvalue reference to const overload f("</span> <span class="sy1">&lt;&lt;</span> x <span class="sy1">&lt;&lt;</span> <span class="st0">")<span class="es1">\n</span>"</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">void</span> f<span class="br0">(</span><span class="kw4">int</span><span class="sy3">&amp;&amp;</span> x<span class="br0">)</span>
<span class="br0">{</span>
     <a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> <span class="st0">"rvalue reference overload f("</span> <span class="sy1">&lt;&lt;</span> x <span class="sy1">&lt;&lt;</span> <span class="st0">")<span class="es1">\n</span>"</span><span class="sy4">;</span>
<span class="br0">}</span>
<span class="kw4">int</span> main<span class="br0">(</span><span class="br0">)</span>
<span class="br0">{</span>
    <span class="kw4">int</span> n <span class="sy1">=</span> <span class="nu0">1</span><span class="sy4">;</span>
    <span class="kw4">const</span> <span class="kw4">int</span> x <span class="sy1">=</span> <span class="nu0">2</span><span class="sy4">;</span>
    f<span class="br0">(</span>n<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls f(int&amp;)</span>
    f<span class="br0">(</span>x<span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls f(const int&amp;)</span>
    f<span class="br0">(</span><span class="nu0">3</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// calls f(int&amp;&amp;)</span>
          <span class="co1">// would call f(const int&amp;) if f(int&amp;&amp;) overload wasn't provided</span>
<span class="br0">}</span></pre></div></div>
<p><br> 
</p>
</div>
<p>This allows <a href="move_constructor.html" title="cpp/language/move constructor">move constructors</a>, <a href="move_operator.html" title="cpp/language/move operator">move assignment</a> operators, and other move-aware functions (e.g. <a href="../container/vector/push_back.html" title="cpp/container/vector/push back"><tt>vector::push_back()</tt></a> to be automatically selected when suitable.
</p>
</td>
<td><span class="t-mark-rev t-since-cxx11">(since C++11)</span></td>
</tr>
</table>
<h4>
<span class="editsection">[<a href="http%3A//en.cppreference.com/mwiki/index.php" title="Edit section: Dangling references">edit</a>]</span> <span class="mw-headline" id="Dangling_references">Dangling references</span>
</h4>
<p>Although references, once initialized, always refer to valid objects or functions, it is possible to create a program where the life time of the referred-to object ends, but the reference remains accessible (<i>dangling</i>). Accessing such reference is undefined behavior.
A common example a function returning a reference to an automatic variable:
</p>
<div dir="ltr" class="mw-geshi" style="text-align: left;"><div class="cpp source-cpp"><pre class="de1"><a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span> f<span class="br0">(</span><span class="br0">)</span> <span class="br0">{</span>
    <a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a> str <span class="sy1">=</span> <span class="st0">"Example"</span><span class="sy4">;</span>
    <span class="kw1">return</span> str<span class="sy4">;</span> <span class="co1">// exits the scope of str: str's destructor is called</span>
<span class="br0">}</span>               <span class="co1">// stack storage deallocated</span>
...
<a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a><span class="sy3">&amp;</span> r <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// r is a dangling reference</span>
<a href="../io/basic_ostream.html"><span class="kw1439">std::<span class="me2">cout</span></span></a> <span class="sy1">&lt;&lt;</span> r<span class="sy4">;</span>      <span class="co1">// Undefined: reads from a dangling reference</span>
<a href="../string/basic_string.html"><span class="kw1045">std::<span class="me2">string</span></span></a> s <span class="sy1">=</span> f<span class="br0">(</span><span class="br0">)</span><span class="sy4">;</span> <span class="co1">// Undefined: uses a dangling reference to copy-initialize s</span></pre></div></div>
<p>Note that rvalue references and lvalue references to const extend the lifetimes of temporary objects, see <a href="reference_initialization.html" title="cpp/language/reference initialization">reference initialization</a> for rules and exceptions.
</p>
<p>If the referred-to object was destroyed (e.g. by explicit destructor call), but the storage was not deallocated, a reference to the out-of-lifetime object may be used in limited ways, and may become valid if the object is recreated in the same storage, see <a href="lifetime.html%23Access_outside_of_lifetime" title="cpp/language/lifetime">Access outside of lifetime</a> for details.
</p>
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